Carbon fibers are typically produced from a precursor that can be made from different materials, such as an acrylic, pitch, or cellulose fibers. According to a common processing method, in an initial step, fibrous segments of the precursor material are successively drawn through an oxidation oven, which heats the segments, by means of a circulating flow of hot gas, to a temperature approaching approximately 300° C. An example of such an oven is the Despatch Carbon Fiber Oxidation Oven, available from Despatch Industries, Minneapolis, Minn. A description of such an oven may be found in commonly-assigned U.S. Pat. No. 4,515,561, which is hereby incorporated by reference in its entirety.
In the oxidation oven, the precursor fibers pass back and forth through the oven chamber via a series of rollers. FIG. 1 provides a schematic view of a simple oxidation oven. As can be seen, the fibers move through the oven by passing through the vestibule, through the transition area, through the oven chamber, through the other transition area, and through the other vestibule. At that point, the fibers pass around the roller and back through the oven in the reverse direction. By passing the fibers back and forth all the way through the oven (and perhaps additional ovens), the fibers can be further processed into oxidized fiber.
One noteworthy aspect of such an oxidation oven is that the rollers are positioned outside the oven. The interior of the oven is too hot for conventional rollers, and custom-designing rollers to withstand the heat is generally not practicable. Additionally, there are process benefits to passing the fibers through atmospheric conditions with each pass through the oven. There must be gaps in the sides of the oven to allow the fibers to pass between the rollers and the interior of the oven.
However, the ability to pass the fibers freely between the rollers and the interior of the oven must be balanced with the desire to isolate the oven chamber from the atmosphere surrounding the oven, including inhibiting relatively cold atmospheric air from seeping into the chamber through the gaps, as such air can adversely affect how the fibers are processed.
In the oxidation oven shown in FIG. 1, the vestibules and the transition areas aim to isolate the oven chamber from the surrounding atmosphere. Conventional transition areas include return ducts. Return ducts direct chamber gas that is near the transition area back to the chamber's heater for recirculation into the chamber. A byproduct of the reaction that occurs inside the chamber is HCN gas. Any gas that flows past the transition area enters the vestibule. Vestibules are under negative pressure. Exhaust from the vestibule flows into abatement equipment to remove the HCN before venting to the atmosphere.
While such vestibules and transition areas are satisfactory, they have limitations. First, processing the vestibule's entire volume of gas through the abatement equipment is not efficient. Moreover, the vestibules and the transition areas do not meaningfully address the problem of cooler atmospheric air entering the oven chamber through the gaps.